During the ordovician period of the early paleozoic era the first jawless fishes appeared on earth these organisms belong to a group known as

Vertebrates.

Explanation:

The second period of the Paleozoic Era, called Ordovician, lasted from 495 to 443 million years. The fauna of this period is composed of several invertebrates, primitive fish and algae that were extinct in about 60% of genera and 25% of families at the end of the period. However, the first jawless fish that appeared on land during this period are classified as vertebrates.

The Ordovician also marks the end of the glacial period that began at the end of the Proterozoic Eon.

"d.) carbon dioxide and hydrogen sulfide"

answer : haversian

[explanation: -(compact bone is organized as parallel columns, known as haversian systems, which run lengthwise down the axis of long bones. these columns are composed of lamellae, concentric rings of bone, surrounding a central channel, or haversian canal, that contains the nerves, blood vessels, and lymphatic system of the bone)]